Scanning Microscopy


Theoretical methods for describing charge transfer processes in atom-surface collisions will be reviewed. Special emphasis will be on the resonant tunneling mechanism, which normally is expected to be the dominant decay mechanism of excited states near metal surfaces. Recent theoretical calculations have shown that the lifetimes for excited atomic states near metal surfaces can be much longer than what previously has been believed. This finding has important consequences for the interpretation and modeling of charge transfer processes in atom-surface scattering events. In particular, it means that excitations in a desorbing species formed at the time of impact or near the surface may survive the passage through the surface region.

In the region close to the surface, it is important to describe the hybridization between the atomic and the surface levels as well as effects of impurities on the local electronic structure. It is shown that such effects can be particularly strong when alkali atoms are coadsorbed on the surface. At finite alkali coverage, the energies of atomic levels will appear corrugated along the surface. It is shown that this effect can drastically influence the probability for a charge exchange process in an atom-surface scattering event.

A dynamical theory for describing ion/atom-surface charge exchange processes that takes into account the low tunneling rates as well as non-image-like level shirts and lateral corrugations of the surface potential at small atom-surface separations is presented. The results are applied to recent experimental sputtering, desorption and ion-surface neutralization data. Good agreement between experiments and theoretical predictions is found indicating that the theoretical model is accurate.